The primary reasons for sur lie ageing are usually based on stylistic goals: to enhance the structure and mouthfeel of a wine, give it extra body (an impact of polysaccharides on astringency), and increase the aromatic complexity, flavour/aroma depth and length. Lees also absorb oxygen, assisting in maintaining a slow and controlled oxidation during maturation.
Lees stirring can increase the release of yeast compounds into the wine bulk. Stirring can result in a creamy, viscous mouth feel, and can enhance flavour complexity.

Some of the compounds from broken down yeast cells also contribute to wine in the following ways:

polysaccharides contribute a roundness and volume to the palate

mannoproteins can bind with anthocyanins and tannins to increase colour stability and decrease astringency

the nutrients released from the dead yeast cells assist the growth of malolactic bacteria

increased palate length, attributed to the late release of particular volatile compounds in the polysaccharide network of the fruit and yeast

they can assist in protection from oxidation of particular fruit aroma compounds.

the process of proteolysis, whereby proteins are hydrolysed to amino acids (which can act as flavour precursors, possibly enhancing flavour complexity) and peptides (which travel through the yeast cell walls causing an increase in nitrogen content).

the release of amino acids and nucleic acids can enhance flavours and complex aromas, particularly at the end of the palate

they yield a sweetness when binding with wood phenols and organic acids

they modify wine esters and wood aromas

they provide a natural fining, reducing more yellow colours in whites

they improve protein stability (current research suggests that lees do this by producing an extra mannoprotein (polysaccharidic molecules which constitute ~35% of the yeast's cell) which prevents polymerisation of tannins, pigments and volatiles; and more of this compound is released when the temperature is increased, and with greater contact time and lees stirring frequency)

Lees and MLF

Lees contact is often run parallel with malolactic fermentation. The goals of each technique are similar in some respects (e.g. creating a wine which is generally perceived as softer). Lees in the presence of MLF tends to lead to lower diacetyl ("buttery") concentrations, since the yeast metabolise the diacetyl to the non-aromatic compounds acetoin and 2,3-butanediol.

Potential risks and prevention

Potential problems

It is vitally important to conduct regular sensory evaluations of wines undergoing sur lie to monitor them for the presence of any reductive aromas. These may include hydrogen sulphide (the smell of rotten eggs), yeast autolysis without stirring (usually resulting in overly yeasty aromas), or off flavours in general. A low oxygen content (for example, in large tanks) can be particularly dangerous.

Hydrogen sulphide (H2S), with its characteristic rotten egg aroma, is the more common problem when sur lie ageing. It results from chemically reduced (the opposite of oxidation) sulphur dioxide and can be detected at about 1 ppb (1 part in 1 billion). There are also a number of reduced sulphur groups and alcohols. While some add complexity to wine aroma, too much can permanently ruin a wine as the reactions are non-reversible.

Mercaptans pose a more serious problem. These are sulphur compounds corresponding to wine alcohols with a thiol (-SH) group rather than a hydroxy (-OH) group. Ethyl mercaptan smells like burnt rubber, or garlic and methyl mercaptan is often likened to rotten or cooked cabbage. The sensory threshold is again about 1 ppb. Mercaptans can be oxidised to disulphides. Methyl mercaptan oxidises to form dimethyl (di)sulfide and smells like onion or cooked cabbage with a sensory threshold of about 30 ppb.

Prevention and cure

Wines with minimal reductive tones that have not yet further complexed may be treated with some oxidation (e.g. racking) to bring the wine back into a more oxidative and healthier aromatic state. Due to its greater tendency to develop off aromas and flavours, gross (or heavy) lees may be avoided in favour of fine lees. Lees than contains vegetal matter should be avoided in any case. Lees that has been in barrels for two or more months for example, are much less likely to acquire reductive aromas.

Preventing hydrogen sulphide formation involves sensible sulphur dioxide usage, avoiding fruit with elemental sulphur, providing sufficient nutrients for yeast prior to fermentation, and use of a yeast which produces little hydrogen sulphide during fermentation. Wines that do not produce significant hydrogen sulphide during fermentation are less likely to encounter problems later.

Sulphur dioxide usage should not be overlooked here. If large quantities of sulphur dioxide are added to the must, they may later bind with acetaldehyde during fermentation. Acetaldehyde is normally reduced during fermentation to form ethanol. If there is a lack of acetaldehyde present, other juice components such as sulphate may be reduced instead, leading to hydrogen sulphide.
Additionally, sulphur dioxide can convert hydrogen sulphide to elemental sulphur, which may subsequently be reduced back to hydrogen sulphide.

Sur lie and SO2

It is worth noting that the early use of SO2 increases the number of compounds that bind with later SO2 additions. Excessive SO2 tends to oppose wood/oak flavours and stunts flavour development, whereas insufficient SO2 favours premature ageing and flattens the wine.
When malolactic fermentation (MLF) is to take place, extra care must be taken with sur lie ageing since no (or very little) sulphur dioxide will/should be present to protect the wine. Sur lie ageing in this situation does provide some assistance against oxidation (through oxidative buffering and the fact that some dissolved carbon dioxide will remain from fermentation), but it also provides a window of opportunity for the development of unwanted bacteria. The most secure approach is to inoculate with a MLF culture early on in the ageing process, or perhaps during alcoholic fermentation itself. Some winemakers, however, feel that a period of non-SO2 lees contact post MLF can contribute further complexity to wines. For more information regarding the connection between MLF, sulphur dioxide and lees contact, see the MLF article.
In the case of wines which are not to under go MLF, or where a wine has already completed MLF, it is helpful to maintain suitable sulphur dioxide levels while sur lie ageing. See the Sulphur Dioxide article for details.

Lees control: types and quantity

Lees stirring is usually conducted on "fine lees" and not "gross/heavy lees".
Heavy lees can be defined as comprising of particles that are typically 100 microns to 2 millimetres. They tend to settle within 24 hours (without pectin). This definition certainly includes fruit debris. Fruit debris can impart off and herbaceous aromas and flavours and can combine with SO2 blocking its antimicrobial and antioxidant properties. Heavy lees may yet further be defined as those substances continually forming in wine. In this case they may also comprise of yeast, bacteria, precipitated tannins/colouring matter/colloids, and particles formed from fining treatments. These can release undesirable bitter substances into the wine.

Light lees comprise of particles that are typically of size one micron to a few tens of microns. They tend to remain in suspension for longer than 24 hours after they have been agitated. They are comprised of yeast and lactic bacteria (produced towards the end of alcoholic fermentation and malolactic fermentation, respectively). Light lees is favoured for sur lie and stirring, but still poses risks of producing reductive aromas/flavours if inappropriately managed.

Separating the gross/heavy lees from the must provides the first opportunity to control the content of lees in the must/wine (the process is called "débourbage" in French). The first racking following alcoholic fermentation gives an opportunity to control the amount of fine lees to be used for sur lie ageing. Choosing the timing of heavy lees separation is an important aspect of lees management. Many winemakers make a judgement on the amount of lees to include in the bulk wine (or whether to include any at all) based on its taste (whether "clean" or "dirty" smelling) at either of these two important stages.

Lees and yeast

The influence of the compounds from broken down yeast cells depends on the individual yeast strains (though the release kinetics of mannoproteins appears to be independent of yeast strain). Different results should therefore be expected from different yeasts. While this influence is not important as other features (such as lees quantity and type control), it may still be taken into consideration.

Lees stirring (Bâtonnage)

Bâtonnage is the French term for stirring the settled lees back into the wine.

Method of stirring

Any method which redistributes the lees thoroughly provides an adequate method of stirring, but different methods contribute differently to wine.
On a moderate scale (such as a single barrel), this can be achieved by the use of a steel rod with a paddle at the end which can be placed in the tank or barrel and spun with an electric drill attached.
On a small scale, a food turntable can be used which allows the process to be controlled externally.
It is important to note the oxygen exposure that each method exerts on the wine, since this will significantly impact wine development.

Intensity and frequency

Lees should be stirred thoroughly, bringing all lees into suspension. The more lees there is, the more frequently stirring should be conducted.

Oxidation and vessel

Oxygen uptake during lees stirring plays an important part in wine development. For example, oxidative stirring increases acetaldehyde concentrations and may increase the acetic acid concentration. The oxygen uptake of a wine under stirring should be factored into the decisions made on maturing schedules.
The vessel type and size in which stirring is conducted should also be noted.

Benefits of stirring

Stirring redistributes the previously mentioned desirable compounds (such as polysaccharides and mannoproteins) of dead cells into the wine mass and, in the case of barrel ageing, re-exposes the wine to the wood at the bottom of the barrel.
Lees absorb oxygen, in the absence of which the wine will become reductive. Bâtonnage, or lees stirring, helps prevent this by redistributing the lees (especially at reductive-point zones) into the wine mass and potentially exposing the wine to some oxygen.
Living yeast cells can enzymatically consume ruptured dead yeast cells. Excessive pressure (such as experienced in large tanks) on dead cells is a primary cause for yeast cell rupture. This is called yeast autolysis and is the process that the traditional Champenoise method employs to gain yeasty/bready notes in Champagne style wines. Yeast autolysis is not usually a feature of sur lie ageing since this process tends to occur at time scales greater than those usually used for sur lie (i.e. around a year as opposed to the several months of sur lie ageing).

Stirring schedules

General schedule

Lees stirring schedules, and the total time spent on lees, vary widely depending on winemaking set-up, wine type and style, and winemaker. It is important to find a schedule that suits your own set-up, wine style and individual wine.

Stirring is often done every 2-3 days during the last stages of fermentation (helps prevent sepsis in the lees) and then every 1-3 months once fermentation is complete. However, sur lie ageing in newer barrels might involve more frequent stirring to integrate and re-expose the wood into the wine. The total time spent on lees varies depending on the wine and style in question. An 8-10 month time frame is typical, however, some wines will experience only a few weeks on lees, whereas others may spend 18 or even 24 months.

Common schedules

Common schedules, beginning after alcoholic fermentation, include:
Once-twice each week for 6 months.
Once each week for the first 4 weeks, then once fortnightly for the next 6 weeks, and monthly thereafter.

Some schedules involve a period of stirring followed by a period on non-stirring contact, for example:
Twice each week for 10 weeks, followed by 6-12 months of non-stirring contact.
Thrice each week for 6 months, followed by 3-4 months of non-stirring contact.

Extreme schedules

Some of the winemakers of top white Burgundy provide perfect examples of extensive sur lie schedules:

Under Guillaume de Castelnau, the wines at Dom. Génot-Boulanger (Meursault, Burgundy) are fermented with fine lees (and possibly gross lees if it is "healthy") before being stirred every 7-10 days and "led toward an oxidative state, and then brought back into a reductive state prior to MLF." Following MLF they are placed in stainless steel "to gain body".

Of course, not all white Burgundy producers use extreme schedules. Alain Coche of Dom. Alain Coche-Bizouard (Meursault), for example, lees stirs once every 8 days for about 4 months and once every month thereafter.

Generally speaking, those who bulk age sur lie for longer periods of time tend to stir less, and vice versa.

Bâtonnage and character/delicacy/finesse: counter opinions

Many Burgundian producers argue that (excessive) stirring can be detrimental to wine quality. They feel that lees stirring tends to reduce a wine's individual character, delicacy, and finesse, and that wines made using this technique are not capable of ageing as long as those made without it.

Jean-Marc Roulot, winemaker at Dom. Guy Roulot (Meursault, Burgundy), is more restrained when it comes to bâtonnage. He believes that excessive stirring of the lees can destroy wine delicacy, refinement and flavour intricacy, and therefore conducts bâtonnage sparingly.

Dom. François Jobard (Meursault, Burgundy), who makes "some of Meursault's tightest and long-lived wines" according to Pierre-Antoine Rovani [Parker, 2000], goes a step further. Jobard leaves his wines on gross lees for over a year, rarely stirring the lees and never stirring the fine lees while his wines are in barrel. He believes that stirring results in a faster evolution in the bottle. He reasons that this is because residual carbon dioxide (remaining from fermentation), which would otherwise protect wines against oxidation, is lost during stirring.